11.1.1 Metafont output at any size

We described above how to get Metafont output at a size of 10pt.
To generate a GF file for a font foo at a different size, assign
to designsize on the command line, as follows:

mf '\mode:=localfont; designsize:=integer; input foo

For example, if localfont corresponds to a 300dpi
device, and you specify `designsize:=6', this command creates
`foo.180gf', i.e., a 40% reduction from
`foo.300gf'.

In some cases, it may be more convenient to specify a magnification
factor than a new point size. (For example, this is the case if you are
enlarging or reducing an entire document by some constant factor, as
with TeX's \magnification command.) You can do this by
assigning to mag:

mf '\mode:=localfont; mag:=real; input foo

By default, mag is 1.0. You can also assign to both mag
and designsize. For example, if you set designsize to 5
and mag to 4, the output will be a 20pt font.

Although the Metafont language allows nonlinear scaling of fonts, so
that the 6pt font would not simply be a reduced version of the
10pt font, BZRto cannot take advantage of this sophistication.
The reason is that BZR files specify a single set of outlines, and the
nonlinear scaling cannot be deduced from that. Perhaps we will extend
the programs someday to handle interpolation between outlines of
different sizes.